Detection of smuggled nuclear and radiological weapons is an urgent national priority. All nuclear and radiological threat materials emit gamma rays or neutrons or both. However, radioactive materials may be shielded in various ways, and obfuscated among benign cargo, making them difficult to detect. An advanced detector such as a directional gamma ray or neutron detector is needed to reveal threat items rapidly and reliably. A directional detector would localize the threat material if present, and would also speed up the inspection process at shipping ports generally. The directional information would greatly enhance the statistical power of a radiation scan, since even a few particles coming from the same point in the cargo would reveal a hidden source, whereas a non-directional detector would require hundreds or thousands of additional detections just to raise a suspicion that some kind of radiation might be somewhere nearby. In addition, the entire inspection process would be speeded up, reducing inspection times and entry waits at shipping ports. Clean loads could be cleared more quickly. Any secondary inspections, when necessary, could then use the directional information as a time-saving starting point.
Since gamma rays are neutral, they are detected only when they interact with matter. Gamma interactions include photoelectric absorption in which the gamma is absorbed and a photoelectron is emitted, or Compton scattering which generates a Compton electron and a scattered gamma ray, or electron-positron pair production. In each case, the energetic electron (or positron) can be detected in a charged-particle detector such as a scintillator, which generates light when traversed by the energetic electrons. Gamma rays are attenuated by material, notably by high-density, high-Z material (Z being the atomic number) such as lead.
Neutrons are a critical signature of plutonium, the primary component of most nuclear weapons. The neutrons from spontaneous fission of plutonium typically have an energy of about 1 MeV, with a spread in energies from about 0.5 to about 5 MeV. Neutrons in that energy range interact with matter primarily by scattering from atomic nuclei in the matter. For most nuclei, the scattering can be either elastic or inelastic depending on the nucleus and other factors. But hydrogen is an exception because the hydrogen nucleus, a proton, has no excited states. Hence every n-p (neutron-proton) scattering in the MeV energy range is an elastic scattering. Energetic neutrons can be detected by n-p scattering, with recoil proton detection in a scintillator or other charged-particle detector. Low energy or thermalized neutrons can be detected by neutron capture in a nucleus, such as 10B or 6Li, which then emits energetic ions that can be measured by scintillators. Thus a multi-purpose scintillator such as a boron-loaded plastic scintillator can detect gamma rays, energetic neutrons, and thermalized neutrons according to the various interaction processes listed.
Prior-art directional gamma ray detectors have usually employed collimators such as pinhole or multi-channel collimators, or coded-aperture masks. Such systems are notoriously inefficient since most of the gamma rays are absorbed in the collimator. Various semi-collimators such as movable baffles and shields have been offered, but they provide very limited angular resolution, truncated field of view, low efficiency, or cumbersome and expensive mechanicals. Other prior-art directional detectors use elongate scintillators or semiconductor detectors shaped to provide an angular dependence on detection, generally resulting in low efficiency and/or poor angular resolution. Tracking-type detectors form an image of the gamma-generated electron track or the neutron-generated recoil proton track. Tracking detectors such as proportional chambers, spark chambers, and bubble chambers are big, heavy, expensive, complex systems unsuitable for field applications such as a vehicle scanner.
What is needed is a compact, solid-state detector that indicates the direction of the incident particle, providing sufficient angular precision to localize the source, rapidly and at high efficiency, and preferably with low cost.